Biophysical Foundations of Electrodiagnostics: Bioelectricity and Excitable Tissue

Introduction to

Biopotential Measurement and Excitation

(Bioelectromagnetism – Jakko Malmivuo, Robert Plonsey)

Electrical activity of excitable cells

• Biopotential• An electric potential

that is measured between points in living cells, tissues, and organisms, and which accompanies all biochemical processes.

• Describes the transfer of information between and within cells

• Excitability of nerve cell

• Excitatory

• Inhibitory

Bioelectric Function of the Nerve Cell

The membrane voltage (Vm) of an excitable cell is defined as the potential at the inner surface (Φi) relative to that at the outer (Φo) surface of the

membrane,

i.e. Vm = (Φi) - (Φo).

Transmembrane potentials according to Theodore H.

Bullock.

Nerve Impulse or Muscle Impulse

Mechanism behind biopotentials 1/2

mVVm 100...70

The number of ions flowing through an open channel >106/sec

Body is an inhomogeneous volume conductor and these ion fluxes create

measurable potentials on body surface

Mechanism behind biopotentials 2/2

Potential difference between excited and unexcited regions of an axon would cause small currents, now called local circuit currents

Saltatory Conduction

Activation Currents in Cardiac Tissue

Electrocardiography (ECG)

• Measures galvanically the electric activity of the heart

• Amplitude: 1-5 mV

• Bandwidth: 0.05-100 Hz

1. Atrial depolarization

2. Ventriculardepolarization

3. Ventricular repolarization

Electroencephalography (EEG)

• Amplitude: 0.001-0.01 mV• Bandwidth: 0.5-40 Hz

• Errors:• Thermal RF noise• 50/60 Hz power lines• Blink artifacts and similar

• Typical applications:• Sleep studies• Seizure detection• Cortical mapping

Electromyography (EMG)

• Measures the electric activity of active muscle fibers

• Electrodes are always connected very close to the muscle group being measured

• Rectified and integrated EMG signal gives rough indication of the muscle activity

• Needle electrodes can be used to measure individual muscle fibers

• Amplitude: 1-10 mV

• Bandwidth: 20-2000 Hz

• Main sources of errors are 50/60 Hz and RF interference

• Applications: muscle function, neuromuscular disease, prosthesis

Electrooculography (EOG)

• Electric potentials are created as a result of the movement of the eyeballs

• Potential varies in proportion to the amplitude of the movement

• In many ways a challenging measurement with some clinical value

• Amplitude: 0.01-0.1 mV

• Bandwidth: DC-10 Hz

• Primary sources of error include skin potential and motion

• Applications: eye position, sleep state, vestibulo-ocular reflex

The biopotential amplifier

• Small amplitudes, low frequencies, environmental and biological sources of interference etc.

• Essential requirements for measurement equipment:• High amplification

• High differential gain, low common mode gain high CMRR

• High input impedance

• Low Noise

• Stability against temperature and voltage fluctuations

• Electrical safety, isolation and defibrillation protection

Application-specific requirements

• ECG amplifier• Lower corner frequency 0.05 Hz, upper 100Hz

• Safety and protection: leakage current below safety standard limit of 10 uA

• Electrical isolation from the power line and the earth ground

• Protection against high defibrillation voltages

• EEG amplifier• Gain must deal with microvolt or lower levels of signals

• Components must have low thermal and electronic noise @ the front end

• Otherwise similar to ECG

• EMG amplifier• Slightly enhanced amplifier BW suffices

• Post-processing circuits are almost always needed (e.g. rectifier + integrator)

• EOG amplifier• High gain with very good low frequency (or even DC) response

• DC-drifting electrodes should be selected with great care

• Often active DC or drift cancellation or correction circuit may be necessary

Defibrillation Protection

• Measuring instruments can encounter very high voltages

• E.g. 1500…5000V shocks from defibrillator

• Front-end must be designed to withstand these high voltages

1. Resistors in the inputleads limit the current

3. Protection against much higher voltages

is achieved withlow-pressure gasdischarge tubes

(e.g. neon lamps)

(note: even isolationcomponents such as

transformers andoptical isolators need

these spark gaps)Discharge @ ~100V

2. Diodes or Zener diodesprotect against high

voltages

Discharge @ 0.7-15V

Types of electrodes

Uses

Ag-AgCl Ambulatory or long term use

Gold EEG, sometimes in EMG

Metal or carbon Electrical stimulation and impedance plethysmography

Needle Small signals such as motor unit potentials

Electrodes - Basics

• Skin preparation by abrasion or cleansing

• Placement close to the source being measured

• Placement above bony structures where there is less muscle mass

• Distinguishing features of different electrodes:• How secure? The structure and the use of strong but less irritant adhesives

• How conductive? Use of noble metals vs. cheaper materials

• How prone to artifact? Use of low-junction-potential materials such as Ag-AgCl

• If electrolytic gel is used, how is it applied? High conductivity gels can help reduce the junction potentials and resistance but tend to be more allergenic or irritating

Baseline drift due to thechanges in junction

potential or motion artifactsChoice of electrodes Muscle signal

interference Placement

Electromagneticinterference Shielding

Thank You

Ag-AgCl, Silver-Silver Chloride Electrodes

• The most commonly used electrode type

• Silver is interfaced with its salt silver-chloride

• Choice of materials helps to reduce junction potentials• Junction potentials are the result of the dissimilar

electrolytic interfaces

• Electrolytic gel enhances conductivity and also reduces junction potentials

• Typically based on sodium or potassium chloride, concentration in the order of 0.1 M weak enough to not irritate the skin

• The gel is typically soaked into a foam pad or applied directly in a pocket produced by electrode housing

• Relatively low-cost and general purpose electrode

• Particularly suited for ambulatory or long term use

Gold Electrodes• Very high conductivity suitable for low-noise meas.

• Inertness suitable for reusable electrodes

• Body forms cavity which is filled with electrolytic gel

• Compared to Ag-AgCL: greater expense, higherjunction potentials and motion artifacts

• Often used in EEG, sometimes in EMG

Conductive polymer electrodes

• Made out of material that is simultaneously conductive and adhesive

• Polymer is made conductive by adding monovalent metallic ions

• Aluminum foil allows contact to external instrumentation

• No need for gel or other adhesive substance

• High resistivity makes unsuitable for low-noise meas.

• Not as good connection as with traditional electrodes

Metal or carbon electrodes

• Other metals are seldom used as high-quality noblemetal electrodes or low-cost carbon or polymericelectrodes are so readily available

• Historical value. Bulky and awkward to use• Carbon electrodes have high resistivity and are noisier

but they are also flexibleand reusable• Applications in electrical stimulation and impedance plethysmography

Needle electrodes• Obviously invasive electrodes

• Used when measurements have to be taken from the organ itself

• Small signals such as motor unit potentials can be measured

• Needle is often a steel wire with hooked tip